The present invention relates to vortex flowmeters and, more particularly, to the use of two vortex flowmeters in tandem.
Vortex flowmeter systems are used in the industrial process control field for measuring flow rate of a fluid. Vortex flowmeters are typically inserted in a flow pipe or conduit that carries the fluid to be measured. Industry applications include petroleum, chemical, pulp and paper, mining and materials, oil and gas. The fluids to be measured are often hazardous, and may be flammable or explosive. The fluids may also include condensate, deposits and other properties that can make flow measurements difficult. In these applications, safety and measurement accuracy are critical.
The operating principal of a vortex flowmeter is based on a phenomenon of vortex shedding known as von Karman effect. As fluid passes a bluff body, it separates and generates small eddies or vortices that are shed alternately along and behind each side of the bluff body. These vortices cause areas of fluctuating pressure that are detected by a sensor. While the frequency of vortex generation is essentially proportional to fluid velocity, this proportionality varies slightly with the conduit Reynolds number. The conduit Reynolds number is a function of the fluid density, the fluid viscosity, the fluid velocity, and the inside diameter of the conduit. The slight variations in the proportionality induce non-linearity in the frequency output of a vortex flowmeter as a function of flow rate.
Vortex flowmeters are also susceptible to characteristic changes after installation due to coating of the bluff body, erosion of the bluff body and other dimensional changes which may cause drift in the measured frequency or other fault conditions. Drift in the measured output is particularly difficult to detect without recalibrating the meter.
Vortex flowmeter systems have been proposed that use two vortex flowmeters in series with one another, such that the outputs can be compared. However, the use of two vortex flowmeters can adversely affect each other due to the vortices generated by the upstream flowmeter.
Thus, there is a need for an improved dual flow metering system which has greater linearity and in which fault conditions are more easily detectable on-line during normal operation.